A passive optical network (Passive Optical Network, PON) technology is currently one of the most widely applied fiber to the home (Fiber To The Home, FTTH) technologies. Existing PONs include a broadband passive optical network (Broadband Passive Optical Network, BPON), a gigabit-capable passive optical network (Gigabit-capable Passive Optical Network, GPON), and an Ethernet passive optical network (Ethernet Passive Optical Network, EPON).
What is shown in FIG. 1 is a diagram of a networking architecture of using a traditional PON system for access. The PON is used to connect a user terminal to a transport network, and then connect it through the transport network to a core network, that is, a digital video (Video) network, an Internet (Internet), and a public switched telephone network (Public Switched Telephone Network, PSTN).
The most basic components of a PON system include: an optical line terminal (Optical Line Terminal, OLT), an optical network unit (Optical Network Unit, ONU), and an optical distribution network (Optical Distribution Network, ODN), and so on. The OLT and a passive optical splitter are connected by a trunk fiber. The optical splitter realizes point-to-multi-point optical power distribution, and is connected to multiple ONUs through multiple branch fibers. The trunk fiber, the passive optical splitter, and the branch fibers are between the OLT and the ONU and are collectively called the ODN. A direction from the OLT to the ONU is called a downstream direction, and a direction from the ONU to the OLT is called an upstream direction.
A time division multiple address (Time Division Multiple Address, TDMA) multiplexing manner is adopted in the upstream direction, and each ONU sends an upstream data stream only at a timeslot that is specified by the OLT. In the downstream direction, a time division multiplexing (Time Division Multiplexing, TDM) broadcasting manner is adopted. The OLT sends a downstream data stream to each ONU, and uses a specific identity to indicate that which ONU each timeslot belongs to. Optical signals that carry downstream data streams of all ONUs are divided into several parts at the optical splitter of the ODN, and arrive at each ONU through each branch fiber. Each ONU collects the data that belongs to itself according to a corresponding identity, and discards data of other timeslots.
In the traditional PON system, the coverage of the OLT over the ONU does not exceed 20 km, and the quantity of ONUs that are connected through the optical splitter is relatively small. As a result, the quantity of OLTs in a traditional PON networking architecture is relatively large, location areas are scattered and remote, management and maintenance is very inconvenient, and device investment and maintenance cost are relatively high.
With the emergence of the next-generation optical access network, carriers demand extending the coverage of the PON, and therefore, a long reach PON (long reach PON, LR-PON) reach extension technology is brought forward. A reach extension solution of the PON system, where the reach extension solution is based on an optical transport network (Optical Transport Network, OTN) technology and a wavelength division multiplexing (Wavelength Division Multiplexing, WDM) technology, has advantages such as reducing fibers, easy to maintain and manage, and high reliability, thereby drawing wide attention and motivation from the industry. Especially, a reach extension solution of an OTN-based GPON system (GPON over OTN) is successfully brought into standards, and is adopted by multiple carriers.
What is shown in FIG. 2 is a schematic architecture diagram of an OTN-based GPON reach extension system. An OTN device is added between the OLT and the optical splitter of the ODN, and the wavelength division multiplexing (Wavelength Division Multiplexing, WDM) technology is used to realize bidirectional data transmission. The OTN device transparently maps (bit mapping) upstream and downstream data frames of the GPON to OTU-k frames, transmits them through an OTN network to a remote device, and then obtains original GPON upstream and downstream data frames through de-mapping, thereby realizing reach extension of the GPON system.
It is supposed that the waiting time from the time when the OLT starts to send the downstream data stream to the time when the upstream data stream that is sent by the ONUs is received is T. Then, a maximum value of the waiting time T that can be tolerated by the OLT is converted into the kilometers of the fiber, that is, a maximum logical distance which is of the OLT and which is defined by a protocol. For example, the maximum logical distance which is of the OLT and which is defined by a current GPON standard is 60 km. In an existing PON reach extension solution, an extension distance is limited by the maximum logical distance which is of the OLT and which is defined by the protocol. If the PON system is extended to exceed the maximum logical distance that is specified by the protocol, the OLT cannot receive the upstream data stream that is sent by the ONUs in the foregoing specified time T, thereby causing that the OLT cannot receive the upstream data stream normally.